This invention relates to moisture sensors for detecting moisture such as rainfall and dew as well as other matter such as dirt, dust, salt film and the like on windows. The invention is especially adapted for vehicle windows to control accessories such as windshield wipers and the like. More particularly, the invention concerns an optical sensing apparatus and assembly for mounting on one side of a window panel for detecting moisture and other matter on the opposite side of the window panel while maximizing the area sensed and minimizing the overall assembly size.
Various types of moisture sensing apparatus have been used on vehicles to control accessories such as windshield wipers. Optical moisture sensors have been found particularly useful since they require no special coatings or other structure on the exterior of the window or vehicle for physically contacting rainfall or moisture. One such device employs an infrared emitter and detector which senses infrared energy emitted from inside the windshield or window and refracted and reflected back at decreased levels when moisture or other matter is present on the outside of the windshield to a detector also mounted on the interior of the vehicle. Such a sensor is disclosed in U.S. Pat. No. 4,798,956 and includes an emitter which emits pulses of infrared energy toward the inside surface of a window at an incident angle. The radiant energy is refracted into the window at the inside air/window surface and reflected off the air/outside window surface back through the window where it is again refracted and detected by a detector/sensor positioned along a reflection angle equal to the incident angle. The emitter and detector are spaced and positioned at predetermined locations and angles all as set forth in U.S. Pat. No. 4,798,956.
As an improvement over the moisture sensing device of U.S. Pat. No. 4,798,956, an optical moisture sensing device which compensates various environmental conditions is disclosed in co-pending, commonly assigned U.S. Pat. No. 4,871,927. That improved device provides stable operation over a range of operational temperatures, over the lifetime of the unit, and in varying types of environmental and ambient light conditions. Moreover, a mounting apparatus is provided which secures the device in proper geometrical relationship adjacent a window surface and provides a protective enclosure for the sensing device and associated wiring. The mount is detachably fastened to the window for removal and replacement when repairs or window replacement are necessary.
Use of the above sensing devices revealed that the area of the window being sensed was relatively small, and that the probability of moisture impinging on the sensed area was likewise small, especially in light rain conditions. Thus, in conditions where a relatively small and scattered number of raindrops strike the vehicle and its window areas, the probability of such a drop falling on the sensed area and being indicated to control the desired vehicle accessory (such as the windshield wipers) often prevented operation of the wipers until long after significant rain had begun to fall.
The prior sensing devices were also relatively difficult and tedious to assemble and were relatively difficult to properly test prior to final assembly. Further, the mounting assembly for securing the sensing device to the windshield required precise, accurate positioning on the inside windshield/window surface in order to orient the sensing unit properly when mounted. This required extra care and time during installation on vehicles thereby increasing costs for use of the device.
Therefore, a need was recognized for an improved moisture sensing device of the optical type which would maximize the window area being sensed while minimizing the size of the sensing assembly. It was also desired to provide reduced complexity and difficulty in manufacturing the sensing device while providing opportunities for testing various components of the overall assembly prior to incorporation in the final unit. Further, it was desired to provide an improved mounting structure which could be more quickly installed while allowing easier orientation of the sensing device in its operational position.